Article ID: 2020-018
The Baiu front (BF) is generally formed in May in the western North Pacific. Using objective analysis data from 1979 to 2014 (36 years), this study investigated the interannual variability of Baiu frontal activity (BFA) in May (BFA-M). In May, seasonal enhancement of warm and moist southerlies from the tropics climatologically establishes the BF as a large-scale quasi-stationary front. The strength of the southerlies from the tropics also controls the interannual variability of BFA-M. The anomalous large-scale circulation centered around Taiwan, which can be interpreted as a moist Rossby wave from the equatorial Kelvin–Rossby wave packet in the western tropical Pacific, modifies the strength of the southerlies. The equatorial Kelvin–Rossby wave packet, which is identified as the equatorial intraseasonal oscillation (ISO), propagates eastward from the Indian Ocean to the western Pacific.
The interannual variability of BFA-M has a biennial tendency, which stands in contrast with the three-year or four-year variation period of the El Niño/Southern Oscillation (ENSO). The biennial tendency is characterized by a zonal tripole distribution of sea surface temperature anomalies in the tropical Pacific, with corresponding anomalous Walker circulations. The induced anomaly fields are suitable for confining the disintegration of the equatorial Kelvin–Rossby wave packet in the western tropical Pacific and guiding the following northwestward propagation of the moist Rossby wave. With the phase reversal of ISO, the biennial tendency remains in the western part of the BF from May to mid-June, although the ENSO controls the BFA in the central part of the BF in June. This study proposes that the equatorial ISO in the Indian Ocean in April can be an indicator of BFA-M strength in the western North Pacific.